The present invention relates to a magnetic recording medium such as a magnetic recording tape used in audio instruments, video instruments and computers. More particularly, the invention relates to a magnetic recording medium having an improved backing layer on the reverse side of the film base to the magnetic layer with which the output fluctuation in the playing of the magnetic recording medium can be remarkably decreased and the runout of the travelling magnetic recording tape can be minimized.
Needless to say, magnetic recording tapes are widely used as a means for information recording in the field of electronics including audio and video technologies as well as computer technologies. Along with the progress of the information processing technology by use of magnetic recording tapes, the amount of information recorded on such magnetic recording tapes is rapidly increasing year by year so that it is an urgent requirement for magnetic recording tapes that the recording density of the information on the tape should be as high as possible.
Usually, short-wavelength recording is undertaken to ensure high-density recording but this way of recording is subject to the problem of dropout, which is a phenomenon of infidelity in the readout of the information recorded on the magnetic recording tape by skipping pulses indispensable for the reproduction of the information with high fidelity.
Such a phenomenon of dropout is caused mainly by the instantaneous or momentary increase of the spacing loss between the magnetic recording tape and the magnetic head of the playback instrument. The spacing loss therebetween is expressed by the formula 54.6d/.lambda.[dB], in which d is the distance between the tape and the magnetic head and .lambda. is the wavelength of recording. As is understood from this formula, the spacing loss is much larger in the short-wavelength recording for high density recording than in the long-wavelength recording so that, in the former case, even an extremely small foreign body adhering to the surface of the tape may cause remarkable increase in the spacing loss resulting in the phenomenon of dropout.
As is explained in the above, the phenomenon of dropout takes place when a tiny foreign body produced in the manufacturing and/or in the course of use of the magnetic recording tape is present on the surface of the tape to increase the spacing between the tape and the magnetic head. Such undesirable foreign bodies on the tape surface are formed by the magnetic particles having fallen off the surface of the magnetic coating layer on the tape due to the deterioration of the coating layer by the repeatedly applied stress in the frequent recording and playing of the recording tape or by the particles of dust or debris of the film base necessarily shaved off during travelling of the tape to be electrostatically attracted on to the surface of the film base and then transferred to the surface of the magnetic coating layer.
In order to prevent the above described drawbacks, in particular, by the latter mechanism, several methods have been proposed including a method of reducing electrostatic charge on the surface by providing an antistatic coating on the surface of the tape reverse to the magnetic coating layer with an antistatic agent or with a coating composition prepared by dispersing an electroconductive powder such as carbon black or graphite powder in an organic binder and a method of providing a protective backing layer to the reverse side surface with a coating composition prepared by dispersing silicon dioxide or other reinforcing filler in an organic binder to decrease shave-off of the film base of the travelling magnetic recording tape.
The above described methods are indeed effective to some extent to prevent gradual increase of the dropout in the repeated use of a magnetic recording tape. The effect of these methods so far obtained is, however, far from satisfactory and it is eagerly desired to develop an improved magnetic recording tape with complete suppression of dropout.
In particular, it has been unexpectedly noted that the method of providing a protective backing layer is not so effective in decreasing the phenomenon of dropout in the early stage of the life of a magnetic recording tape before so many numbers of repeated travelling, i.e. recording and playing, of the tape. It is usual that the coating of the backing layer on the film base is performed after the magnetic coating layer has been provided followed by calendering since, when the order is reversed, i.e. when the backing layer is first provided on one of the surfaces of the film base and then the magnetic coating layer is formed on the other surface followed by calendering, no complete smoothing effect of the surface of the magnetic coating layer can be obtained by the calendering because the ruggedness in the backing layer is transferred or copied to the magnetic coating layer in the calendering.
The binder material for the backing layer is usually a thermosetting resin because it is an essential requirement that the binder material should be strong enough not to cause increase of the phenomenon of dropout even after so many numbers of repeated travelling of the tape regardless of the type of the filler incorporated therein which may be an electroconductive powder such as carbon black and graphite powder or other inorganic particulate materials. In the case of using a thermosetting resin as the binder material of the backing layer, the coating composition for the backing layer is first applied to the surface of the film base followed by winding of the thus coated tape into a roll and then the thermosetting resin is subjected to the curing treatment as on the rolled tape. Needless to say, the backing layer containing the thermosetting resin as formed by the application of a coating composition to the film base has no sufficient mechanical strengths before curing of the resin so that, when the tape is wound up into a roll bringing the backing layer and the magnetic coating layer into direct contact with each other, the particles of the carbon black, graphite powder or other inorganic filler impregnating the backing layer may sometimes be transferred to the surface of the magnetic coating layer in contact with the backing layer before curing. The detailed investigations undertaken by the inventors for the mechanism of the phenomenon of dropout led to a discovery that the phenomena of dropout in playing of the tape and fill-up of the surface of the magnetic head are sometimes attributable to the filler particles transferred from the uncured backing layer to the magnetic coating layer. This is the very reason for the disadvantage that the phenomenon of dropout is not decreased so much as expected at the initial stage of the life of a magnetic recording tape by providing a backing layer to the tape even though the backing layer is considerably effective in suppressing the increase of the dropout after repeated travelling of the tape. In other words, the phenomenon of dropout takes place considerably even at the very beginning stage of the use of a new magnetic recording tape since the tape as prepared per se is not free from certain particulate materials adhering to the surface of the magnetic coating layer as transferred from the backing layer. Presumably, the effect of the backing layer to suppress the increase of the dropout is exhibited only after a number of repeated travelling of the tape with the reinforcing and antistatic effects thereof. The situation may be similar when a thermoplastic resin is used as the binder resin for the backing layer.